Optical filter and optical instrument

ABSTRACT

In an optical filter, a thin film is formed by laminating low refractive index layers alternatingly from a substrate side with high refractive index layers. The thin film is provided with first, second, and third laminated portions. In the first laminated portion, the refractive indices of the high refractive index layers become gradually higher. In the second laminated portion, the refractive indices of the high refractive index layers are substantially equal to the highest refractive index of the high refractive index layers constituting the first laminated portion. In the third laminated portion, the refractive indices of the high refractive index layers become gradually lower, and the refractive indices of the low refractive index layers are substantially equal to the lowest refractive index of the low refractive index layers constituting the second laminated portion. The low refractive index layers of the first and second laminated portions have a low refractive index layer uniform portion or a low refractive index layer decreasing portion.

RELATED APPLICATION INFORMATION

This application is a divisional application of prior U.S. applicationSer. No. 10/917,479 filed Aug. 13, 2004, which claims priority toJapanese Patent Application No. 2003-299224, filed Aug. 22, 2003,Japanese Patent Application No. 2003-299225, filed Aug. 22, 2003,Japanese Patent Application No. 2003-299226, filed Aug. 22, 2003, andJapanese Patent Application No. 2003-354027, filed Oct. 14, 2003, thecontents of each of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical filter and opticalinstrument.

2. Description of Related Art

A fluorescence microscope, which is an optical instrument used whenobserving biological specimens, is able to analyze the structure andnature of a specimen, such as a cell that has been treated with dye, byobserving fluorescent light emitted by the specimen when excitationlight is irradiated thereon.

In order to perform the latest genomic analysis, there is a need toobserve, for example, both fluorescent light having a peak at 526 nm andexcitation light having a wavelength of 502 nm. In this case, becausethe wavelength of the excitation light is close to the wavelength of thefluorescent light, in order for the fluorescent light to be moreefficiently detected, an optical filter that cuts out the excitationlight using a stopband and that allows light of the fluorescent lightobservation wavelength to pass through using a transmission band is usedas an extremely important key part in order to determine the sensitivityand accuracy of the fluorescent light measurement.

In this optical filter, properties that permit a sharp rise in thespectral characteristics at boundaries between transmission bands andstopbands, and that also allow substantially 100% of light to betransmitted in the transmission band are demanded. Furthermore, in thetransmission band, it is desirable that there are no cyclic variations(i.e., ripples) in the transmittance in response to increases ordecreases in the wavelength.

A minus filter, which is an optical filter that cuts out light in apredetermined wavelength band and allows light of other wavelengths topass through in this manner, is manufactured, as is shown in FIG. 33A,using a multi-layer film in which layers having a high refractive indexand layers having a low refractive index are laminated alternatingly ona substrate. Here, the horizontal axis shows the optical thickness whilethe vertical axis shows the film refractive index. In addition, in FIG.33B the relationship between the transmittance and the wavelength oflight that passes through a film during construction of the film isshown as a spectral characteristic. Here, the optical thickness isdetermined by multiplying the physical thickness of the film by theindex of the film.

The optical filter is able to make the rise at boundaries betweentransmission bands and stopbands sharper as the number of theaforementioned layers is increased. However, the problem arises that asthe number of layers is increased, the ripples in the transmission bandsalso increase. Moreover, as is shown in FIG. 34A, it is possible todesign a film in which ripples are reduced by changing the opticalthickness of each layer, however, as is shown in FIG. 34B, it isdifficult to do away with ripples completely.

In contrast to this, as is shown in FIG. 35A, if the refractive index ofthe film is changed cyclically and continuously in the optical thicknessdirection such that the refractive index distribution thereof is formedinto what is known as a “wavelet” configuration, then, as is shown inFIG. 35B, it is possible to fundamentally do away with ripples in thetransmission band. Moreover, for example, as is shown in FIG. 36A, FIG.36B, FIG. 37A, FIG. 37A, FIG. 38A, and FIG. 38A, various types ofstructures have been proposed in which a continuous refractive indexdistribution is divided into stages and approximated.

SUMMARY OF THE INVENTION

The present invention is an optical filter formed by a substrate and athin film that is formed on the substrate, wherein the thin film isformed by laminating low refractive index layers whose refractive indexis relatively low alternatingly from the substrate side with highrefractive index layers whose refractive index is relatively high, andwherein the thin film is provided with a first laminated portion, asecond laminated portion that is adjacent to this first laminatedportion, and a third laminated portion that adjacent to this secondlaminated portion, and wherein in the first laminated portion, therefractive indices of the high refractive index layers becomes graduallyhigher approaching the substrate, in the second laminated portion, therefractive indices of the high refractive index layers are substantiallyequal to the highest refractive index of the high refractive indexlayers constituting the first laminated portion, and in the thirdlaminated portion, the refractive indices of the high refractive indexlayers become gradually lower from the second laminated portion side,and the refractive indices of the low refractive index layers aresubstantially equal to the lowest refractive index of the low refractiveindex layers constituting the second laminated portion, and wherein thelow refractive index layers of the first and second laminated portionshave at least one of a low refractive index layer uniform portion inwhich the refractive indices are substantially uniform, or a lowrefractive index layer decreasing portion in which the refractiveindices become gradually lower approaching the substrate.

The present invention is an optical filter formed by a substrate and athin film that is formed on the substrate, wherein the thin film isformed by laminating low refractive index layers whose refractive indexis relatively low alternatingly from the substrate side with highrefractive index layers whose refractive index is relatively high, andwherein the thin film is provided with a first laminated portion, asecond laminated portion that is adjacent to this first laminatedportion, a third laminated portion that is adjacent to this secondlaminated portion, and a fourth laminated portion that is adjacent tothis third laminated portion, and wherein in the first laminatedportion, the refractive indices of the high refractive index layersbecomes gradually higher approaching the substrate, and the refractiveindices of the low refractive index layers become gradually lowerapproaching the substrate, in the second laminated portion, therefractive indices of the high refractive index layers become graduallyhigher from the highest refractive index of the high refractive indexlayers constituting the first laminated portion, and the refractiveindices of the low refractive index layers are substantially equal tothe lowest refractive index of the low refractive index layersconstituting the first laminated portion, in the third laminatedportion, the refractive indices of the high refractive index layersbecome gradually lower from the highest refractive index of the highrefractive index layers constituting the second laminated portion, andthe refractive indices of the low refractive index layers aresubstantially equal to the refractive indices of the low refractiveindex layers constituting the second laminated portion, and in thefourth laminated portion, the refractive indices of the high refractiveindex layers become gradually lower from the lowest refractive index ofthe high refractive index layers constituting the third laminatedportion, and the refractive indices of the low refractive index layersbecome gradually higher from the low refractive index layersconstituting the third laminated portion.

The present invention is an optical filter formed by a substrate and athin film that is formed on the substrate, wherein the thin film isformed by laminating low refractive index layers whose refractive indexis relatively low alternatingly from the substrate side with highrefractive index layers whose refractive index is relatively high, andwherein the thin film is provided with a first laminated portion, asecond laminated portion that is adjacent to the first laminatedportion, and a third laminated portion that is adjacent to the secondlaminated portion, and wherein in the first laminated portion, therefractive indices of the high refractive index layers becomes graduallyhigher approaching the substrate, in the second laminated portion, therefractive indices of the high refractive index layers are substantiallyequal to the highest refractive index of the high refractive indexlayers constituting the first laminated portion, and in the thirdlaminated portion, the refractive indices of the high refractive indexlayers become gradually lower from the second laminated portion side,and wherein the refractive indices of the low refractive index layersconstituting the first through third laminated portions aresubstantially equal to the refractive index of the substrate, and anabsolute value of a refractive index gradient of the high refractiveindex layers constituting the first laminated portion is different froman absolute value of a refractive index gradient of the high refractiveindex layers constituting the third laminated portion.

The present invention is an optical filter formed by a substrate and athin film that is formed on the substrate, wherein the thin film isformed by laminating low refractive index layers whose refractive indexis relatively low alternatingly from the substrate side with highrefractive index layers whose refractive index is relatively high, andwherein the thin film is provided with a first laminated portion, asecond laminated portion that is adjacent to the first laminatedportion, and a third laminated portion that is adjacent to the secondlaminated portion, and wherein in the first laminated portion, therefractive indices of the low refractive index layers become graduallylower approaching the substrate side, in the second laminated portion,the refractive indices of the low refractive index layers aresubstantially equal to the lowest refractive index of the low refractiveindex layers constituting the first laminated portion, and in the thirdlaminated portion, the refractive indices of the low refractive indexlayers become gradually higher from the second laminated portion side,and wherein the refractive indices of the high refractive index layersconstituting the first through third laminated portions aresubstantially equal to the refractive index of the substrate.

The present invention is an optical filter formed by a substrate and athin film that is formed on the substrate, wherein the thin film isprovided with laminated portions having predetermined refractive indexprofiles, and wherein the refractive index profiles are formed bylaminating low refractive index layers whose refractive index isrelatively low alternatingly from the substrate side with highrefractive index layers whose refractive index is relatively high, andwherein the refractive index profiles are represented by a firstlaminated portion, a second laminated portion that is laminated on thesubstrate side of the first laminated portion, and a third laminatedportion that is laminated on the substrate side of the second laminatedportion, and wherein in the first laminated portion, the refractiveindices of the high refractive index layers become gradually higherapproaching the substrate side, in the second laminated portion, therefractive indices of the high refractive index layers are substantiallyequal to the highest refractive index of the high refractive indexlayers constituting the first laminated portion, and in the thirdlaminated portion, the refractive indices of the high refractive indexlayers become gradually lower approaching the substrate side, andwherein an optical thickness of the high refractive index layers and anoptical thickness of the low refractive index layers are different fromeach other.

The present invention is an optical filter formed by a substrate and athin film that is formed on the substrate, wherein the thin film isprovided with laminated portions having predetermined refractive indexprofiles, and wherein the refractive index profiles are formed bylaminating low refractive index layers whose refractive index isrelatively low alternatingly from the substrate side with highrefractive index layers whose refractive index is relatively high, andwherein the refractive index profiles are represented by a firstlaminated portion and a second laminated portion that is adjacent to thesubstrate side of the first laminated portion, and wherein in the firstlaminated portion, the refractive indices of the high refractive indexlayers become gradually higher approaching the substrate side, and therefractive indices of the low refractive index layers become graduallylower approaching the substrate side, and in the second laminatedportion, the refractive indices of the high refractive index layersbecome gradually lower approaching the substrate side, and therefractive indices of the low refractive index layers become graduallyhigher approaching the substrate side, and wherein an optical thicknessof the high refractive index layers and an optical thickness of the lowrefractive index layers are different from each other.

The present invention is an optical filter formed by a substrate and athin film that is formed on the substrate, wherein the thin film isprovided with laminated portions having a first refractive index profileand a second refractive index profile, and wherein the first and secondrefractive index profiles are formed by laminating low refractive indexlayers whose refractive index is relatively low alternatingly from thesubstrate side with high refractive index layers whose refractive indexis relatively high, and wherein the first refractive index profile andthe second refractive index profile are continuous, and wherein each ofthe first refractive index profile and the second refractive indexprofile is represented by a first laminated portion, a second laminatedportion that is laminated on the substrate side of the first laminatedportion, and a third laminated portion that is laminated on thesubstrate side of the second laminated portion, and wherein in the firstlaminated portion, the refractive indices of the high refractive indexlayers become gradually higher approaching the substrate side, in thesecond laminated portion, the refractive indices of the high refractiveindex layers are substantially equal to the highest refractive index ofthe high refractive index layers constituting the first laminatedportion, and in the third laminated portion, the refractive indices ofthe high refractive index layers become gradually lower approaching thesubstrate side, and wherein at least one of an optical thickness of thehigh refractive index layers in the first refractive index profile, anoptical thickness of the low refractive index layers in the firstrefractive index profile, an optical thickness of the high refractiveindex layers in the second refractive index profile, and an opticalthickness of the low refractive index layers in the second refractiveindex profile is different from the others.

The present invention is an optical filter formed by a substrate and athin film that is formed on the substrate, wherein the thin film isprovided with laminated portions having a first refractive index profileand a second refractive index profile, and wherein the first and secondrefractive index profiles are formed by laminating low refractive indexlayers whose refractive index is relatively low alternatingly from thesubstrate side with high refractive index layers whose refractive indexis relatively high, and wherein the first refractive index profile andthe second refractive index profile are continuous, and wherein each ofthe first refractive index profile and the second refractive indexprofile is represented by a first laminated portion and a secondlaminated portion that is adjacent to the substrate side of the firstlaminated portion, and wherein in the first laminated portion, therefractive indices of the high refractive index layers become graduallyhigher approaching the substrate side, and the refractive indices of thelow refractive index layers become gradually lower approaching thesubstrate side, in the second laminated portion, the refractive indicesof the high refractive index layers become gradually lower approachingthe substrate side, and the refractive indices of the low refractiveindex layers become gradually higher approaching the substrate side, andwherein at least one of an optical thickness of the high refractiveindex layers in the first refractive index profile, an optical thicknessof the low refractive index layers in the first refractive indexprofile, an optical thickness of the high refractive index layers in thesecond refractive index profile, and an optical thickness of the lowrefractive index layers in the second refractive index profile isdifferent from the others.

The present invention is an optical filter formed by a substrate and athin film that is formed on the substrate, wherein the thin film isformed by laminating low refractive index layers whose refractive indexis relatively low alternatingly from the substrate side with highrefractive index layers whose refractive index is relatively high, andwherein the thin film is provided with a first laminated portion, asecond laminated portion that is adjacent to the first laminatedportion, and a third laminated portion that is adjacent to the secondlaminated portion, and wherein in the first laminated portion, therefractive indices of the high refractive index layers become graduallyhigher approaching the substrate, in the second laminated portion, therefractive indices of the high refractive index layers are substantiallyequal to the highest refractive index of the high refractive indexlayers constituting the first laminated portion, and in the thirdlaminated portion, the refractive indices of the high refractive indexlayers become gradually lower from the second laminated portion side,and wherein lamination patterns in which the first laminated portion,the second laminated portion, and the third laminated portion arearranged in this sequence approaching the substrate are repeated twiceor more in a thickness direction of the film, and the optical thicknessof each lamination pattern is different.

The present invention is an optical filter formed by a substrate and athin film that is formed on the substrate, wherein the thin film isformed by laminating low refractive index layers whose refractive indexis relatively low alternatingly from the substrate side with highrefractive index layers whose refractive index is relatively high, andwherein the thin film is provided with a first laminated portion, asecond laminated portion that is adjacent to the first laminatedportion, and a third laminated portion that is adjacent to the secondlaminated portion, and wherein in the first laminated portion, therefractive indices of the low refractive index layers become graduallylower approaching the substrate, in the second laminated portion, therefractive indices of the low refractive index layers are substantiallyequal to the lowest refractive index of the low refractive index layersconstituting the first laminated portion, and in the third laminatedportion, the refractive indices of the low refractive index layersbecome gradually higher from the second laminated portion side, andwherein lamination patterns in which the first laminated portion, thesecond laminated portion, and the third laminated portion are arrangedin this sequence approaching the substrate are repeated twice or more ina thickness direction of the film, and the optical thickness of eachlamination pattern is different.

The present invention is an optical filter provided with a substrate anda thin film that is formed on the substrate, wherein the thin film hasan outermost layer portion that is in contact with an optical mediumwhose refractive index is lower than the refractive index of thesubstrate, and has laminated portions having predetermined refractiveprofiles, and wherein the refractive index profiles are formed bylaminating low refractive index layers alternatingly with highrefractive index layers approaching the substrate side, and wherein therefractive indices of the low refractive index layers is higher than therefractive index of the optical medium, and the refractive indices ofthe high refractive index layers is relatively higher than therefractive index of the low refractive index layers, and wherein theoutermost layer portion is provided with an outermost low refractiveindex layer, a first outermost high refractive index layer that islaminated on the outermost low refractive index layer, and a secondoutermost high refractive index layer that is laminated on the firstoutermost high refractive index layer, and wherein the refractive indexof the outermost low refractive index layer is higher than therefractive index of the optical medium, the refractive index of thefirst outermost high refractive index layer is higher than therefractive index of the outermost low refractive index layer, and therefractive index of the second outermost high refractive index layer ishigher than the refractive index of the first outermost high refractiveindex layer, and wherein the refractive index profile is represented bya first laminated portion that is laminated on the outermost layerportion side, a second laminated portion that is laminated on thesubstrate side of the first laminated portion, and a third laminatedportion that is laminated on the substrate side of the second laminatedportion, and wherein in the first laminated portion, the refractiveindices of the high refractive index layers are higher than the secondoutermost high refractive index layer, and become gradually higherapproaching the substrate side, in the second laminated portion, therefractive indices of the high refractive index layers are substantiallyequal to the highest refractive index of the high refractive indexlayers constituting the first laminated portion, and in the thirdlaminated portion, the refractive indices of the high refractive indexlayers become gradually lower approaching the substrate side.

BRIEF DESCRIPTION THE DRAWINGS

FIG. 1 is a view showing an outline of the fluorescence microscopeaccording to the first embodiment of the present invention.

FIGS. 2A and 2B are graphs showing a film structure and spectralcharacteristics of an absorption filter in a first embodiment of thepresent invention.

FIGS. 3A and 3B are graphs showing a film structure and spectralcharacteristics of an absorption filter in a second embodiment of thepresent invention.

FIGS. 4A and 4B are graphs showing a film structure and spectralcharacteristics of an absorption filter in a third embodiment of thepresent invention.

FIGS. 5A and 5B are graphs showing a film structure and spectralcharacteristics of an absorption filter in a fourth embodiment of thepresent invention.

FIGS. 6A and 6B are graphs showing a film structure and spectralcharacteristics of an absorption filter in a fifth embodiment of thepresent invention.

FIGS. 7A and 7B are graphs showing a film structure and spectralcharacteristics of an absorption filter in a sixth embodiment of thepresent invention.

FIGS. 8A and 8B are graphs showing a film structure and spectralcharacteristics of an absorption filter in a seventh embodiment of thepresent invention.

FIGS. 9A and 9B are graphs showing a film structure and spectralcharacteristics of an absorption filter in an eighth embodiment of thepresent invention.

FIGS. 10A and 10B are graphs showing a film structure and spectralcharacteristics of an absorption filter in another example of the eighthembodiment of the present invention.

FIGS. 11A and 11B are graphs showing a film structure and spectralcharacteristics of an absorption filter in another example of the eighthembodiment of the present invention.

FIGS. 12A and 12B are graphs showing a film structure and spectralcharacteristics of an absorption filter in another example of the eighthembodiment of the present invention.

FIGS. 13A and 13B are graphs showing a film structure and spectralcharacteristics of an absorption filter in a ninth embodiment of thepresent invention.

FIGS. 14A and 14B are graphs showing a film structure and spectralcharacteristics of an absorption filter in a tenth embodiment of thepresent invention.

FIGS. 15A and 15B are graphs showing a film structure and spectralcharacteristics of an absorption filter in another example of the tenthembodiment of the present invention.

FIGS. 16A and 16B are graphs showing a film structure and spectralcharacteristics of an absorption filter in another example of the tenthembodiment of the present invention.

FIGS. 17A and 17B are graphs showing a film structure and spectralcharacteristics of an absorption filter in a eleventh embodiment of thepresent invention.

FIGS. 18A and 18B are graphs showing a film structure and spectralcharacteristics of an absorption filter in a twelfth embodiment of thepresent invention.

FIGS. 19A and 19B are graphs showing a film structure and spectralcharacteristics of an absorption filter in a thirteenth embodiment ofthe present invention.

FIGS. 20A and 20B are graphs showing a film structure and spectralcharacteristics of an absorption filter in a fourteenth embodiment ofthe present invention.

FIGS. 21A and 21B are graphs showing a film structure and spectralcharacteristics of an absorption filter in a fifteenth embodiment of thepresent invention.

FIG. 22 is a graph showing a relationship between transmittance and awavelength of the fluorescence microscope of the fifteenth embodiment ofthe present invention.

FIGS. 23A and 23B are graphs showing a film structure and spectralcharacteristics of an absorption filter in another example of thefifteenth embodiment of the present invention.

FIGS. 24A and 24B are graphs showing a film structure and spectralcharacteristics of a conventional absorption filter.

FIGS. 25A and 25B are graphs showing a film structure and spectralcharacteristics of an absorption filter in another example of thefifteenth embodiment of the present invention.

FIGS. 26A and 26B are graphs showing a film structure and spectralcharacteristics of a conventional absorption filter.

FIGS. 27A and 27B are graphs showing a film structure and spectralcharacteristics of an absorption filter in a sixteenth embodiment of thepresent invention.

FIGS. 28A and 28B are graphs showing a film structure and spectralcharacteristics of a conventional absorption filter.

FIGS. 29A and 29B are graphs showing a film structure and spectralcharacteristics of an absorption filter in another example of aseventeenth embodiment of the present invention.

FIGS. 30A and 30B are graphs showing a film structure and spectralcharacteristics of a conventional absorption filter.

FIGS. 31A and 31B are graphs showing a film structure and spectralcharacteristics of an absorption filter in an eighteenth embodiment ofthe present invention.

FIGS. 32A and 32B are graphs showing a film structure and spectralcharacteristics of a conventional absorption filter.

FIGS. 33A and 33B are graphs showing a film structure and spectralcharacteristics of a conventional absorption filter.

FIGS. 34A and 34B are graphs showing a film structure and spectralcharacteristics of a conventional absorption filter.

FIGS. 35A and 35B are graphs showing a film structure and spectralcharacteristics of a conventional absorption filter.

FIGS. 36A and 36B are graphs showing a film structure and spectralcharacteristics of a conventional absorption filter.

FIGS. 37A and 37B are graphs showing a film structure and spectralcharacteristics of a conventional absorption filter.

FIGS. 38A and 38B are graphs showing a film structure and spectralcharacteristics of a conventional absorption filter.

DETAILED DESCRIPTION OF THE INVENTION

While preferred embodiments of the invention have been described andillustrated above, it should be understood that these are exemplary ofthe invention and are not to be considered as limiting. Additions,omissions, substitutions, and other modifications can be made withoutdeparting from the spirit or scope of the present invention.Accordingly, the invention is not to be considered as limited by theforegoing description and is only limited by the scope of the appendedclaims.

Next, the first embodiment of the present invention will be describedwith reference made to FIGS. 1, 2A, and 2B.

The optical instrument according to the present embodiment is shown inFIG. 1. The optical instrument in FIG. 1 is a fluorescence microscope.The fluorescence microscope 10 is provided with an excitation filter 11,a dichroic mirror 12, an absorption filter (i.e., an optical filter) 13,an ocular lens 14, and an objective lens 15.

The excitation filter 11 is placed on an optical path of the lightsource 16. As a result of this placement, it selectively allows onlyspecific wavelengths out of the light that is generated from the lightsource 16 to pass through in the form of excitation light.

The dichroic mirror 12 is placed on the opposite side from the lightsource 16 sandwiching the excitation filter 11 so as to bend the opticalaxis at an angle of 45°. Accordingly, excitation light that has passedthrough the excitation filter 11 is irradiated via the objective lens 15onto a specimen 17 such as, for example, a biological cell. As a resultof this irradiation fluorescent light is generated from the specimen 17.Because the generated fluorescent light has a different wavelength fromthat of the excitation light, it passes through the dichroic mirror 12and arrives at the observing side.

The ocular lens 14 and objective lens 15 form an observation opticalsystem. Using this observation optical system, an image of theaforementioned fluorescent light can be observed.

The absorption filter 13 is formed by a glass substrate 18, a thin film19, and an incident medium 18′. Here, the thin film 19 is formed on topof the substrate 18. In addition, the incident medium 18′ is provided ontop of the thin film 19. This absorption filter 13 has opticalcharacteristics that selectively allow only the aforementionedfluorescent light to pass through. The incident medium 18′ is formed bya member (for example, a glass plate) having the same refractive indexas that of the substrate 18.

The absorption filter 13 will now be described. In FIG. 2A, theabsorption filter is shown by the symbol 13-1. As is shown in FIG. 2A,the thin film 19 is formed by low refractive index layers 20 and highrefractive index layers 21. The refractive indices of the highrefractive index layers 21 have a relatively high refractive index incomparison with the refractive indices of the low refractive indexlayers 20. The low refractive index layers 20 and the high refractiveindex layers 21 are laminated alternatingly from the substrate 18 side.

The thin film 19 is also formed by a first laminated portion 22, asecond laminated portion 23, and the third laminated portion 24. Fromthe substrate 18 side towards the incident medium 18′, the laminatedportions are laminated in order of the third laminated portion 24, thesecond laminated portion 23, and the first laminated portion 22. Inaddition, each laminated portion has low refractive index layers 20 andhigh refractive index layers 21.

In the first laminated portion 22, the refractive indices of the highrefractive index layers 21 become gradually higher approaching thesubstrate 18. Furthermore, in the first laminated portion 22, therefractive indices of the low refractive index layers 20 aresubstantially uniform. In this case, the low refractive index layers 20have a low refractive index layer fixed portion.

The second laminated portion 23 is adjacent to the first laminatedportion 22. In the second laminated portion 23, the refractive indicesof the high refractive index layers 21 are substantially uniform. Therefractive indices of the high refractive index layers 21 aresubstantially the same as the highest refractive index of the highrefractive index layers 21 constituting the first laminated portion 22.In the second laminated portion 23, the refractive indices of the lowrefractive index layers 20 are substantially uniform at first.Thereafter, from partway along the refractive indices become graduallylower. Then, once again, the refractive indices become substantiallyuniform at the end. In this way, the second laminated portion 23 has alow refractive index layer fixed portion 26 where the refractive indicesare at first substantially uniform, and a low refractive index layerdecreasing portion 25 where the refractive indices become graduallylower. Low refractive index layer fixed portions 26 are provided on bothsides of the low refractive index layer decreasing portion 25. Note thatportions of a low refractive index layer fixed portion 26 also form thelow refractive index layers 20 of the first laminated portion 22.

The third laminated portion 24 is adjacent to the second laminatedportion 23. In this third laminated portion 24, the refractive indicesof the high refractive index layers 21 become gradually lower from thesecond laminated portion 23 side. Moreover, in the third laminatedportion 24 the refractive indices of the low refractive index layers 20are substantially uniform. The refractive indices of the low refractiveindex layers 20 are substantially the same as the lowest refractiveindex of the low refractive index layers 20 constituting the secondlaminated portion 23.

The low refractive index layers 20 are mainly formed from silicon oxide.In contrast, the high refractive index layers 21 are mainly formed fromniobium oxide.

In the present embodiment, the refractive index of the substrate 18 isset to 1.8. In addition, the refractive indices of the high refractiveindex layers 21 change from 1.81 to 2.2. Moreover, the refractiveindices of the low refractive index layers 20 change from 2.0 to 1.8 inthe low refractive index layer decreasing portion 25.

Here, a central wavelength of a wavelength band in which transmission isblocked is taken as λ. A design wavelength is taken as λ/n (wherein n isan integer). If, for example, n=1, the design wavelength becomes λ. Theoptical thicknesses of the high refractive index layers 21 and the lowrefractive index layers 20 are set at one quarter of the designwavelength.

In the present embodiment, because λ is set to 600 nm, the opticalthicknesses of the high refractive index layers 21 and the lowrefractive index layers 20 are each 150 nm.

Here, the results are shown when a simulation was performed for spectraltransmittance of the absorption filter. In this simulation, in additionto the above structure, the total number of laminated layers is set at68. In addition, a substrate having a refractive index of 2.0 is alsoprovided on the incident medium 18′ side of the thin film 19. In thissimulation, there is also no refractive index dispersion in each layer.The results of a simulation with these parameters and conditions areshown in FIG. 2B.

Next, a method of observation using the fluorescence microscope 10 willbe described.

Light emitted from the light source 16 is irradiated onto the excitationfilter 11. Here, by passing through the excitation filter excitationlight of a specific wavelength is obtained. This excitation light isirradiated into the dichroic mirror 12.

This excitation light is reflected by the dichroic mirror 12 and isirradiated onto the objective lens 15. The excitation light is thencondensed by the objective lens 15 and is irradiated onto the specimen17. At this time, fluorescent light is generated from the specimen 17 bythis irradiation. This fluorescent light is formed into parallel lightvia the objective lens 15. This parallel light arrives at the dichroicmirror 12, passes through the dichroic mirror 12, and arrives at theabsorption filter 13.

The fluorescent light that reaches the absorption filter 13 (i.e., thefilter 13-1 in FIG. 2A) is irradiated from the first laminated portion22. The fluorescent light then passes through the second laminatedportion 23 and the third laminated portion 24 and is once again emittedto the outside from the substrate 18 side.

Reflected light, which includes excitation light in addition tofluorescent light, that is reflected from the specimen 17 is reflectedby the dichroic mirror 12. However, a portion passes through thedichroic mirror 12. As a result, excitation light is also irradiatedonto the absorption filter 13. However, the thin film 19 is providedwith the above described first laminated portion 22 through the thirdlaminated portion 24. Therefore, it is possible to provide theabsorption filter 13 with the spectral transmittance characteristicsshown in FIG. 2B. Here, the absorption filter 13-1 is manufactured suchthat the wavelength band to which the excitation light and the likebelongs is matched to the stopband 28, while the wavelength band towhich the fluorescent light belongs is matched to the transmissionbands. If this type of structure is employed, it is possible to allowfluorescent light only to pass through while preventing excitation lightfrom passing through.

At this time, the optical thicknesses of the high refractive indexlayers 21 and the low refractive index layers 20 are set to one quarterof the design wavelength. If this type of structure is employed, thencontrol of the optical thickness during film formation is simplified. Asa result, a uniform optical thickness can be consistently formed.Accordingly, it is possible to improve the optical characteristics ofthe obtained thin film.

Fluorescent light that is emitted from the absorption filter 13 in thismanner is irradiated onto the ocular lens 14. This fluorescent lightpasses through the ocular lens 14 and is condensed and then arrives atthe observation side.

As is shown, for example, in FIG. 2B, according to this absorptionfilter 13, rises in spectral characteristics at the boundaries betweenthe stopband 28 and the transmission bands 29 can be made extremelysharp. Furthermore, it is possible to almost completely suppress ripples29 a in the transmission bands 29. Moreover, because the film structuresimplifies control during film formation, it is possible to improve theconsistency of the optical characteristics.

Next, the second embodiment will be described with reference made toFIG. 3A. Note that, in the description below, component elements thatare the same as those in the above described embodiment are given thesame symbols and a description thereof is omitted.

The second embodiment differs from the first embodiment in the followingpoint. Namely, in a thin film 40 according to the second embodiment alow refractive index layer decreasing portion 25 is provided at a distalend side of the first laminated portion 22. The second embodiment alsodiffers in that the refractive indices of the low refractive indexlayers of the second laminated portion 23 are substantially uniform.

Namely, the absorption filter 13-2 is provided with the thin film 40. Inthis thin film 40, a low refractive index layer decreasing portion 25 isprovided in the low refractive index layers 20 of the first laminatedportion 22. In addition, a low refractive index layer fixed portion 26is provided moving from the low refractive index layer decreasingportion 25 towards the substrate 18 side in the low refractive indexlayers 20 that constitute the first and second laminated portions 22 and23.

A refractive index variation layer portion is provided in the thin film40. For example, if the vicinity of the boundary between the firstlaminated portion 22 and the second laminated portion 23 is observed, itcan be seen that a layer 41 is present here that is lower than the highrefractive index layers 21 on both sides thereof. Because this layer 41is on the high refractive index side, here, it is taken as a highrefractive index variation layer portion 41. This high refractive indexvariation layer portion 41 is provided on the first laminated portion 22side.

In the same way, a high refractive index variation layer portion 41 isalso provided in the vicinity of the boundary between the secondlaminated portion 23 and the third laminated portion 24. This highrefractive index variation layer portion 41 is provided on the thirdlaminated portion 24 side. Note that the high refractive index variationlayer portions 41 are adjacent to the high refractive index layers 21 onboth sides thereof via low refractive index layers 20.

Low refractive index variation layer portions are also provided in thelow refractive index layers 20. If the vicinity of the boundary betweenthe first laminated portion 22 and the second laminated portion 23 isobserved, it can be seen that a layer 42 is present here in which therefractive index of the low refractive index layer 20 is higher thanthat of the low refractive index layers 20 on both sides thereof. Thisis a low refractive index variation layer portion 42. This lowrefractive index variation layer portion 42 is provided on the firstlaminated portion 22 side. In the same way, a low refractive indexvariation layer portion 42 is also provided in the vicinity of theboundary between the second laminated portion 23 and the third laminatedportion 24. This low refractive index variation layer portion 42 isprovided on the third laminated portion 24 side.

In the present embodiment, the refractive indices of the high refractiveindex layer 21 in the second laminated portion 23 are 2.2. In contrastto this, the refractive indices of the high refractive index variationlayer portions 41 are set to 2.15. Moreover, the refractive indices ofthe low refractive index layers 20 in the second laminated portion 23are set to 1.4. In contrast to this, the refractive indices of the lowrefractive index variation layer portions 42 are set to 1.42.

Again, the results are shown when a simulation was performed forspectral transmittance of the absorption filter. In this simulation, inaddition to the above structure, the total number of laminated layers isset at 75. The thin film 40 is inserted between an incident medium 18′having a refractive index of 1.8 and a substrate 18 having a refractiveindex of 1.4. In this simulation, there is also no refractive indexdispersion in each layer. The results of a simulation with theseparameters and conditions are shown in FIG. 3B.

According to the absorption filter 13-2 of the present embodiment, as isshown, for example, in FIG. 3B, by altering the position of the lowrefractive index layer decreasing portion 25, it is possible to alterthe width of the stopband 28. Furthermore, because both the highrefractive index variation layer portion 41 and the low refractive indexvariation layer portion 42 are provided, it is possible to suppressripples even further. Accordingly, in the same way as in the firstembodiment, it is possible to reduce ripples 29 a in the transmissionbands of fluorescent light and to consistently obtain a sufficientquantity of light.

Next, the third embodiment will be described with reference made to FIG.4A. Note that in the description given below, component elements thatare the same as those of the above described embodiments are given thesame symbols and a description thereof is omitted.

The absorption filter 13-3 of the third embodiment is provided with athin film 50. The thin film 50 is formed by a first laminated portion51, a second laminated portion 52, a third laminated portion 53, and afourth laminated portion 54. From the substrate 18 side towards theincident medium 18′, the laminated portions are formed in order of thefourth laminated portion 54, the third laminated portion 53, the secondlaminated portion 52, and the first laminated portion 51. In addition,each laminated portion has low refractive index layers 20 and highrefractive index layers 21.

In the first laminated portion 51, the refractive indices of the highrefractive index layers 21 become gradually higher approaching thesubstrate 18. In addition, in the first laminated portion 51, therefractive indices of the low refractive index layers 20 becomegradually lower approaching the substrate 18.

The second laminated portion 52 is adjacent to the first laminatedportion 51. In the second laminated portion 52, the refractive indicesof the high refractive index layers 21 become gradually higher from thefirst laminated portion 51 side. In addition, in the second laminatedportion 52, the refractive indices of the low refractive index layers 20are substantially uniform. The refractive indices of these lowrefractive index layers 20 are substantially the same as the lowestrefractive index of the low refractive index layers 20 constituting thefirst laminated portion 51.

The third laminated portion 53 is adjacent to the second laminatedportion 52. In the third laminated portion 53, the refractive indices ofthe high refractive index layers 21 become gradually lower from thesecond laminated portion 52 side. In addition, in the third laminatedportion 53, the refractive indices of the low refractive index layers 20are substantially uniform. The refractive indices of these lowrefractive index layers 20 are substantially the same as the lowestrefractive index of the low refractive index layers 20 constituting thefirst laminated portion 51.

The fourth laminated portion 54 is adjacent to the third laminatedportion 53. In this fourth laminated portion 54, the refractive indicesof the high refractive index layers 21 become gradually lower from thethird laminated portion 53 side. In addition, in the fourth laminatedportion 54, the refractive indices of the low refractive index layers 20become gradually higher from the third laminated portion 53 side.

In the present embodiment, the refractive index of the substrate 18 isset to 1.8. The refractive indices of the high refractive index layers21 change from 1.81 to 2.2. In addition, the refractive indices of thelow refractive index layers 20 change from 1.4 to 1.8.

Here, the results are shown when a simulation was performed for spectraltransmittance of the absorption filter. In this simulation, in additionto the above structure, the total number of laminated layers is set at45. A substrate having a refractive index of 1.8 is also inserted on theincident medium 18′ side of the thin film 32. In this simulation, thereis also no refractive index dispersion in each layer. The results of asimulation with these parameters and conditions are shown in FIG. 4B.

According to the absorption filter 13-3 of the present embodiment, as isshown, for example, in FIG. 4B, it is possible to suppress ripples 29 ain the same way as in the other embodiments. Furthermore, because therefractive indices of the low refractive index layers 20 constitutingthe second and third laminated portions 52 and 53 are substantiallyuniform, control of the refractive indices during film formation is asimple matter.

Next, the fourth embodiment will be described with reference made toFIG. 5A. Note that in the description given below, component elementsthat are the same as those in the above described embodiments are giventhe same symbols and a description thereof is omitted.

The absorption filter 13-4 according to the fourth embodiment isprovided with a thin film 60. The thin film 60 is formed by a firstlaminated portion 22, a second laminated portion 23, and a thirdlaminated portion 24. From the substrate 18 side towards the incidentmedium 18′, the laminated portions are formed in order of the thirdlaminated portion 24, the second laminated portion 23, and the firstlaminated portion 22. In addition, each laminated portion has lowrefractive index layers 20 and high refractive index layers 21.

In the first laminated portion 22, the refractive indices of the highrefractive index layers 21 become gradually higher approaching thesubstrate 18. In addition, in the first laminated portion 22, therefractive indices of the low refractive index layers 20 aresubstantially uniform.

The second laminated portion 23 is adjacent to the first laminatedportion 22. In this second laminated portion 23, the refractive indicesof the high refractive index layers 21 are substantially uniform. Therefractive indices of the high refractive index layers 21 aresubstantially the same as the highest refractive index of the highrefractive index layers 21 constituting the first laminated portion 22.In addition, in the second laminated portion 23, the refractive indicesof the low refractive index layers 20 are substantially uniform.

The third laminated portion 24 is adjacent to the second laminatedportion 23. In this third laminated portion 24, the refractive indicesof the high refractive index layers 21 become gradually lower from thesecond laminated portion 23 side. In addition, in the third laminatedportion 24, the refractive indices of the low refractive index layers 20are substantially uniform.

Note that the refractive indices of the low refractive index layers 20constituting the first through third laminated portions 22 to 24 aresubstantially uniform.

Here, if the refractive index gradients of the high refractive indexlayers 21 are observed, it will be seen that the absolute value of thegradient of the refractive index in the first laminated portion 22 issmaller than the absolute value of the gradient of the refractive indexin the third laminated portion. Moreover, the area where the absolutevalue of the gradient of the refractive index is small is broader incomparison with the area in which the absolute value of the gradient ofthe refractive index is large. In the present embodiment, a thin film 60is formed that provides this type of refractive index gradient.

In the present embodiment, the refractive index of the substrate 18 isset to 1.5. The refractive indices of the high refractive index layers21 change from 1.55 to 2.4. In addition, the refractive indices of thelow refractive index layers 20 are a uniform value of 1.5.

Here, the results are shown when a simulation was performed for spectraltransmittance of the absorption filter. In this simulation, in additionto the above structure, the total number of laminated layers is set at63. A substrate having a refractive index of 1.5 is also inserted on theincident medium 18′ side of the thin film 60. In this simulation, thereis also no refractive index dispersion in each layer. The results of asimulation with these parameters and conditions are shown in FIG. 5B.

Generally, control of the refractive index during film formation is moredifficult than control of the optical thickness. Accordingly, if thereare irregularities in the refractive indices of the refractive indexlayers, it is difficult to obtain the desired optical performance.However, as is the case in the present embodiment, if the refractiveindex gradients of the high refractive index layers 21 are set such thatthe absolute value of the refractive index gradient in the firstlaminated portion 22 is smaller than the absolute value of therefractive index gradient in the third laminated portion, then even ifirregularities occur in the refractive index layer in the firstlaminated portion 22, the effects thereof can be reduced. Accordingly,even if the refractive index varies during film formation, the filmstructure has little deterioration in the optical performance (such asripples or blunting of the sharpness of the rise in the spectralcharacteristics).

Next, the fifth embodiment will be described with reference made to FIG.6A. Note that in the description given below, component elements thatare the same as those in the above described embodiments are given thesame symbols and a description thereof is omitted.

The absorption filter 13-5 of the fifth embodiment is provided with athin film 70. The absorption filter 13-5 differs from that of the fourthembodiment in the following points. Namely, in the thin film 70according to the fifth embodiment, the gradients of the refractiveindices of the high refractive index layers 21 are formed such that theabsolute value of the gradient of the refractive indices of the highrefractive index layers 21 in the first laminated portion 22 is largerthan the absolute value of the gradient of the refractive indices of thehigh refractive index layers 21 in the third laminated portion 24.

Here, the results are shown when a simulation was performed for spectraltransmittance of the absorption filter. In this simulation, in additionto the above structure, the total number of laminated layers is set at63. A substrate having a refractive index of 1.5 is also inserted on theincident medium 18′ side of the thin film 70. In this simulation, thereis also no refractive index dispersion in each layer. The results of asimulation with these parameters and conditions are shown in FIG. 6B.

The thin film 70 of the present embodiment has the same operation andeffects as the thin film 60 of the fourth embodiment.

Next, the sixth embodiment will be described with reference made to FIG.7A. Note that in the description given below, component elements thatare the same as those in the above described embodiments are given thesame symbols and a description thereof is omitted.

The absorption filter 13-6 of the sixth embodiment is provided with athin film 80. The thin film 80 is formed by a first laminated portion22, a second laminated portion 23, and a third laminated portion 24.From the substrate 18 side towards the incident medium 18′, thelaminated portions are formed in order of the third laminated portion24, the second laminated portion 23, and the first laminated portion 22.In addition, each laminated portion has low refractive index layers 20and high refractive index layers 21.

In the first laminated portion 22, the refractive indices of the highrefractive index layers 21 are substantially uniform. In addition, therefractive indices of the low refractive index layers 20 becomegradually lower approaching the substrate 18.

The second laminated portion 23 is adjacent to the first laminatedportion 22. In this second laminated portion 23, the refractive indicesof the high refractive index layers 21 are substantially uniform. Inaddition, in this second laminated portion 23, the refractive indices ofthe low refractive index layers 20 are substantially uniform. Therefractive indices of the low refractive index layers 20 aresubstantially the same as the lowest refractive index of the lowrefractive index layers 20 constituting the first laminated portion 22.

The third laminated portion 24 is adjacent to the second laminatedportion 23. In this third laminated portion 24, the refractive indicesof the high refractive index layers 21 are substantially uniform. Inaddition, in the third laminated portion 24, the refractive indices ofthe low refractive index layers 20 become gradually higher from thesecond laminated portion 23 side.

In the present embodiment, the refractive index of the substrate 18 isset to 1.8. The refractive indices of the high refractive index layers21 is a uniform value of 1.8. In addition, the refractive indices of thelow refractive index layers 20 change from 1.4 to 1.75.

Here, the results are shown when a simulation was performed for spectraltransmittance of the absorption filter. In this simulation, in additionto the above structure, the total number of laminated layers is set at45. A substrate having a refractive index of 1.8 is also inserted on theincident medium 18′ side of the thin film 70. In this simulation, thereis also no refractive index dispersion in each layer. The results of asimulation with these parameters and conditions are shown in FIG. 7B.

According to the absorption filter 13-6 of the present embodiment, as isshown, for example, in FIG. 7B, in the same way as in the abovedescribed embodiments, it is possible to suppress ripples in thetransmission bands and consistently obtain a sufficient quantity oflight. Moreover, because the refractive indices of the high refractiveindex layers 21 are uniform, control of the refractive indices duringfilm formation is a simple matter.

Note that in the above described embodiment, with n=1, the designwavelength is set to 600 nm, which is the same as the centralwavelength, and the optical thicknesses of the high refractive indexlayers 21 and the low refractive index layers 20 are set to one quarterof the design wavelength. However, it is also possible to set n=2, andset the design wavelength at 300 nm. If n=2, the optical thicknesses ofthe high refractive index layers 21 and the low refractive index layers20 are one half of the design wavelength. Even if a thin film is formedwith the optical thicknesses set in this manner, it is possible toobtain an optical filter having exactly the same spectralcharacteristics.

For a central wavelength of 600 nm, the design wavelength is set to600/n (wherein n is an integer) nm. In addition, the optical thicknessesof the high refractive index layers 21 and the low refractive indexlayers 20 are set to n/4 of the design wavelength. Even if a thin filmis formed with the optical thicknesses set in this manner, it ispossible to obtain an optical filter having exactly the same spectralcharacteristics.

Next, the seventh through eleventh embodiments will be described. Thefilter 13 of the seventh through eleventh embodiments differs from thoseof the above described embodiments in the following point. Namely, inthe above described embodiments, the optical thicknesses of the lowrefractive index layers 20 and the high refractive index layers 21 arethe same, however, in the present embodiment these optical thicknessesare different.

The seventh embodiment will now be described with reference made to FIG.8A. Note that, in the description given below, component elements thatare the same as those in the above described embodiments are given thesame symbols and a description thereof is omitted.

The absorption filter 13-7 of the seventh embodiment is provided with athin film 200. As is shown in FIG. 8A, in the thin film 200, lowrefractive index layers 20 whose refractive indices are relatively lowand high refractive index layers 21 whose refractive indices arerelatively high are laminated alternatingly from the substrate 18 side.

A refractive index profile P is represented by a first laminated portion22′, a second laminated portion 23′, and a third laminated portion 24′.The thin film 200 is formed by these three laminated portions. From thesubstrate 18 side towards the incident medium 18′, the laminatedportions are formed in order of the third laminated portion 24′, thesecond laminated portion 23′, and the first laminated portion 22′. Inaddition, each laminated portion has low refractive index layers 20 andhigh refractive index layers 21.

In the first laminated portion 22′, the refractive indices of the highrefractive index layers 21 become gradually higher approaching thesubstrate 18 side. In addition, in the first laminated portion 22′, therefractive indices of the low refractive index layers 20 aresubstantially uniform.

The third laminated portion 24′ is laminated on the substrate 18 side ofthe first laminated portion 22′. In the third laminated portion 24′, therefractive indices of the high refractive index layers 21 becomegradually lower approaching the substrate 18 side. In addition, in thethird laminated portion 24′, the refractive indices of the lowrefractive index layers 20 are substantially uniform.

The second laminated portion 23′ is laminated between the firstlaminated portion 22′ and the third laminated portion 24′. In thissecond laminated portion 23′, the refractive indices of the highrefractive index layers 21 are substantially uniform. The refractiveindices of these high refractive index layers 21 are substantially thesame as the highest refractive index from among the high refractiveindex layers 21 constituting the first laminated portion 22′. Inaddition, in the second laminated portion 23′, the refractive indices ofthe low refractive index layers 20 are substantially uniform.

In the present embodiment, the refractive indices of the low refractiveindex layers 20 are substantially the same in each of the firstlaminated portion 22′, the second laminated portion 23′, and the thirdlaminated portion 24′. Moreover, the optical thickness of the highrefractive index layers 21 and the optical thickness of the lowrefractive index layers 20 are different from each other. In addition,the refractive indices of the low refractive index layers 20 aresubstantially the same as the refractive index of the substrate 18.

Here, the low refractive index layers 20 are mainly formed from siliconoxide. The high refractive index layers 21 are mainly formed fromniobium oxide.

In the present embodiment, the refractive index of the substrate 18 andthe low refractive index layers 20 are set to 1.5. In addition, therefractive indices of the high refractive index layers 21 are changedbetween 1.5 and 2.4.

Here, the results are shown when a simulation was performed for spectraltransmittance of the absorption filter. In this simulation, in additionto the above structure, the total number of laminated layers of the thinfilm 200 is set at 47. This total number of layers is the number oflayers from the substrate 18 side to the final layer that is adjacent tothe incident side medium 18′. The design wavelength is 600 nm. Theoptical thicknesses of the high refractive index layers 21 are 0.25times the design wavelength, while the optical thicknesses of the lowrefractive index layers 20 are 0.5 times the design wavelength.

Moreover, in this simulation, in the refractive index profile P, thereis no refractive index dispersion in the respective layers. The resultswhen transmittance was simulated using these parameters and conditionsare shown in FIG. 8B.

This absorption layer 13-7 is provided with a stopband 22A, a stopband22B, a transmission band 23A, and a transmission band 23B. In thestopband 22A, a central wavelength where transmission is obstructed is450 nm, and the bandwidth where transmittance is 0% is approximately 40nm. In the stopband 22B, a central wavelength where transmission isobstructed is 920 nm, and the bandwidth where transmittance is 0% isapproximately 160 nm. The transmission band 23A and the transmissionband 23B are provided with bandwidths that allow wavelengths other thanthese to be transmitted.

According to this absorption filter 13-7, as is shown, for example, inFIG. 8B, it is possible to form sharp boundaries between the stopbands22A and 22B and the transmission bands 23A and 23B. As a result, it ispossible to increase the amount of transmitted light in the transmissionbands 23A and 23B. Furthermore, in addition to these effects, it ispossible to suppress ripples.

Moreover, the optical thickness ratio between the high refractive indexlayers 21 and the low refractive index layers 20 is set to 1:2 withoutchanging the refractive index in each layer of the refractive indexprofile P. By employing this type of structure, compared with when theyhave the same optical thicknesses (i.e. as is the case conventionally),it is possible to narrow the bandwidth in the stopband 22A wheretransmittance is 0%. Specifically, for example, it is possible tocontract a bandwidth of approximately 130 nm in a conventional stopbandto a bandwidth of approximately 40 nm. In addition, it is possible tomove the central wavelength where transmittance is 0% from 600 nmtowards the shorter wavelength side, for example, to 450 nm. Moreover,it is possible to newly set a stopband 22B in a wavelength band having920 nm as the central wavelength. Accordingly, out of the incident lightthat it is irradiated into the absorption filter, it is possible toprevent the transmission of light in the wavelength bands 22A and 22B,whose central wavelength is 450 nm and 920 nm, and to allow thetransmission of light in the transmission bands 23A and 23B. Therefore,fluorescent dyes are selected that are excited by the light of thewavelength bands 22A and 22B to generate fluorescent light in thetransmission bands 23A and 23B. By employing this type of structure, itis possible to make an observation with little loss of fluorescentlight, and it is possible to make considerable improvements in thedetection sensitivity when measuring fluorescent light. As a result, forexample, in genome analysis and the like, it is possible to improveanalysis accuracy and detection accuracy, and to shorten the observationtime.

Moreover, in the present embodiment, the refractive indices of the lowrefractive index layers 20 are set to the same value as the refractiveindex of the substrate 18. Accordingly, it is possible to suppress lossat boundaries between the substrate 18 and the thin film 200, and tofurther improve the amount of light transmitted in the transmissionbands 23A and 23B.

Next, the eighth embodiment of the present invention will be describedwith reference made to FIG. 9A. Note that, in the description givenbelow, component elements that are the same as those in the abovedescribed embodiments are given the same symbols and a descriptionthereof is omitted.

The eighth embodiment differs from the seventh embodiment in thefollowing point. Namely, in the seventh embodiment, the refractiveindices of the low refractive index layers 20 are the same as that ofthe substrate 18, however, in the eighth embodiment the refractiveindices of the low refractive index layers 20 are gradually changed.

The absorption filter 13-8 of the present embodiment is provided with athin film 210. The refractive index profile 27 of the thin film 210 isrepresented by a first laminated portion 31 and a second laminatedportion 30. The thin film 210 is formed by these two laminated portions.From the substrate 18 side towards the incident medium 18′, thelaminated portions are formed in order of the second laminated portion30 and the first laminated portion 31. In addition, each laminatedportion has low refractive index layers 20 and high refractive indexlayers 21.

In the first laminated portion 31, the refractive indices of the highrefractive index layers 21 become gradually higher approaching thesubstrate 18 side. In addition, in the first laminated portion 31, therefractive indices of the low refractive index layers 20 becomegradually lower approaching the substrate 18 side.

The second laminated portion 30 is adjacent to the first laminatedportion 31 on the substrate 18 side thereof. In this second laminatedportion 30, the refractive indices of the high refractive index layers21 become gradually lower approaching the substrate 18 side. Inaddition, in the second laminated portion 30, the refractive indices ofthe low refractive index layers 20 become gradually higher approachingthe substrate 18 side.

Moreover, the optical thickness of the high refractive index layers 21and the optical thickness of the low refractive index layers 20 aredifferent from each other.

In the present embodiment, as is shown in FIG. 9A, the refractive indexof the substrate 18 is set to 1.8. In addition, the refractive indicesof the low refractive index layers 20 change between less than 1.80 and1.4. Moreover, the refractive indices of the high refractive indexlayers 21 change between 1.82 and 2.2.

Here, the results are shown when a simulation was performed for spectraltransmittance of the absorption filter. In this simulation, in additionto the above structure, the total number of laminated layers of the thinfilm 210 is set at 45. The design wavelength is 800 nm. The opticalthicknesses of the high refractive index layers 21 are 0.25 times thedesign wavelength, while the optical thicknesses of the low refractiveindex layers 20 are 0.125 times the design wavelength.

Moreover, in this simulation, there is no refractive index dispersion inthe respective layers of the thin film 210. The results whentransmittance was simulated using these parameters and conditions areshown in FIG. 9B.

This absorption layer 13-8 is provided with a stopband 33A, atransmission band 35A, and a transmission band 35B. In the stopband 33A,the central wavelength is 610 nm, and the bandwidth where transmittanceis 0% is approximately 60 nm. The transmission bands 35A and 35B areprovided with bandwidths that allow wavelengths other than these to betransmitted.

According to this absorption filter 13-8, as is shown, for example, inFIG. 9B, in the same way as in the seventh embodiment, it is possible toreduce ripples in the transmission bands of fluorescent light.Accordingly, when making an observation of fluorescent light, it ispossible to consistently obtain a sufficient quantity of fluorescentlight. In addition, the optical thicknesses of the high refractive indexlayers 21 and the low refractive index layers 20 are set at a ratio of2:1. As a result, compared with when the optical thicknesses of the twoare the same (i.e. as is the case conventionally), it is possible tonarrow the bandwidth in the stopband 33B where transmittance is 0%.Specifically, for example, it is possible to contract a bandwidth ofapproximately 150 nm in a conventional stopband to a bandwidth ofapproximately 60 nm. In addition, it is possible to move the centralwavelength of the stopband 33A from 800 nm towards the shorterwavelength side, for example, to 610 nm.

Note that, as in the case of the absorption filter 13-8′ shown in FIG.10A, it is possible to change the ratio of the optical thicknesses. Forexample, it is possible to set the optical thicknesses of the highrefractive index layers 21 to 0.125 times the design wavelength, and toset the optical thicknesses of the low refractive index layers 20 to0.25 times the design wavelength. In this case, the optical thicknessratio becomes 1:2, which is the reverse to that shown in FIG. 9A.However, even if this structure is employed, as is shown in FIG. 10B, itis still possible to form a stopband 33A that is the same as that shownin FIG. 9B. In FIG. 10B, the central wavelength and the bandwidth of thestopband 33A are substantially the same as those shown in FIG. 9B. Inthis way, it is possible to obtain the same operation and effects asthose described above even if the ratio of the optical thicknesses ischanged.

It is also possible to change the total number of layers of the thinfilm. A case in which the total number of layers of the thin film is 100and the design wavelength is 1200 nm is shown in FIG. 11A. Here, theoptical thicknesses of the high refractive index layers 21 are 0.025times the design wavelength, while the optical thicknesses of the lowrefractive index layers 20 are 0.5 times the design wavelength.Accordingly, the optical thickness ratio is 1:20. In this case, thespectral transmittance characteristics of the absorption filter 13-8″are as is shown in FIG. 11B. Here, in the stopband 43B, it is possibleto contract a bandwidth where transmittance is 0% from a conventionalapproximately 80 nm to approximately 10 nm. In addition, it is possibleto move the central wavelength from 610 nm to 420 nm. At the same time,it is possible to set a new stopband 43B in which the central wavelengthis 630 nm.

Another example in which the ratio of the optical thicknesses has beenchanged is shown in FIG. 12A. Here, the optical thicknesses of the highrefractive index layers 21 are 0.025 times the design wavelength, whilethe optical thicknesses of the low refractive index layers 20 are 1.0times the design wavelength. In this case, the optical thickness ratiois 1:40. In this case, the spectral transmittance characteristics of theabsorption filter 13-8′″ are as is shown in FIG. 12B. Here, in thestopband 48A, it is possible to contract a bandwidth where transmittanceis 0% from a conventional approximately 30 nm to approximately 5 nm. Inaddition, it is possible to move the central wavelength of the stopband48A to 410 nm. At the same time, it is possible to form a new stopband48B in which the central wavelength is 490 mm, a new stopband 48C inwhich the central wavelength is 615 nm, and a new stopband 48D in whichthe central wavelength is 820 nm.

In these cases as well, is possible to obtain the same operation andeffects as those described above.

In this way, by changing the optical thickness ratio between the highrefractive index layers and the low refractive index layers, it ispossible to move the stopband to an optional position. In addition, itis possible to set the bandwidth of the stopbands to an optional width.At this time, stopbands can be set in a plurality of wavelength bands.

Next, the ninth embodiment will be described with reference made to FIG.13A. Note that, in the description given below, component elements thatare the same as those in the above described embodiments are given thesame symbols and a description thereof is omitted.

The absorption filter 13-9 of the ninth embodiment is provided with athin film 220. The ninth embodiment differs from the above describedembodiments in the following points. Namely, in the ninth embodiment,the thin film 220 is provided with a first refractive index profile P1and a second refractive index profile P2. In addition, a laminatedportion having the first refractive index profile P1 and a laminatedportion having the second refractive index profile P2 are laminatedconsecutively.

The first refractive index profile P1 is represented by a firstlaminated portion 56 and a second laminated portion 58. The secondrefractive index profile P2 is represented by a first laminated portion61 and a second laminated portion 63. The thin film 220 is formed bythese four laminated portions. From the substrate 18 side towards theincident medium 18′, the laminated portions are formed in order of thesecond laminated portion 63, the first laminated portion 61, the secondlaminated portion 58, and the first laminated portion 56. In addition,each laminated portion has low refractive index layers and highrefractive index layers.

The first refractive index profile P1 will now be described. In thefirst laminated portion 56, the refractive indices of the highrefractive index layers 55A become gradually higher approaching thesubstrate 18 side. In addition, in the first laminated portion 56, therefractive indices of the low refractive index layers 55B becomegradually lower approaching the substrate 18 side.

The second laminated portion 58 is adjacent to the first laminatedportion 56 on the substrate 18 side thereof. In this second laminatedportion 58, the refractive indices of the high refractive index layers57A become gradually lower approaching the substrate 18 side. Inaddition, in the second laminated portion 58, the refractive indices ofthe low refractive index layers 57B become gradually higher approachingthe substrate 18 side.

The second refractive index profile P2 will now be described. The firstlaminated portion 61 is adjacent to the second laminated portion 58 onthe substrate 18 side thereof. In this first laminated portion 61, therefractive indices of the high refractive index layers 60A becomegradually higher approaching the substrate 18 side. In addition, in thefirst laminated portion 61, the refractive indices of the low refractiveindex layers 60B become gradually lower approaching the substrate 18side.

The second laminated portion 63 is laminated on the substrate 18 side ofthe first laminated portion 61. In this second laminated portion 63, therefractive indices of the high refractive index layers 62A becomegradually lower approaching the substrate 18 side. In addition, in thesecond laminated portion 63, the refractive indices of the lowrefractive index layers 62B become gradually higher approaching thesubstrate 18 side.

Out of the optical thicknesses of the high refractive index layers 55Aand 57A in the first refractive index variation portion 52, the opticalthicknesses of the low refractive index layers 55B and 57B in the firstrefractive index variation portion 52, the optical thicknesses of thehigh refractive index layers 60A and 62A in the second refractive indexvariation portion 53, and the optical thicknesses of the low refractiveindex layers 60B and 62B in the second refractive index variationportion 53, at least one refractive index variation portion has aoptical thickness that is different from that of the others.

In the present embodiment, as is shown in FIG. 13A, the refractive indexof the substrate 18 is set to 1.9. In addition, the refractive indicesof the high refractive index layers 55A, 57A, 60A, and 62A are changedbetween 1.9 and 2.2. The refractive indices of the low refractive indexlayers 55B, 57B, 60B, and 62B are changed between less than 1.9 and 1.6.

The laminated portion having the first refractive index profile P1 andthe laminated portion having the second refractive index profile P2 eachhave a total of 80 laminated layers. Accordingly, in the thin film 220as a whole, the total number of laminated layers is 160. The designwavelength is 1500 nm. In addition, in the first refractive indexprofile P1, the optical thicknesses of the high refractive index layers55A and 57A and of the low refractive index layers 55B and 57B are eachset to 0.25 times the design wavelength. In the second refractive indexprofile P2, the optical thicknesses of the high refractive index layers60A and 62A and of the low refractive index layers 60B and 62B are eachset to 0.5 times the design wavelength.

Here, the results are shown when a simulation was performed for spectraltransmittance of the absorption filter. In this simulation, there is norefractive index dispersion in the respective layers of the thin film51. The results when transmittance was simulated using these parametersand conditions are shown in FIG. 13B.

This absorption filter 13-9 is provided with a stopband 65A, a stopband65B, a stopband 65C, and transmission bands 66A, 66B, 66C, and 66D. Inthe stopband 65A, a central wavelength where transmission is obstructedis 500 nm, and the bandwidth where transmittance is 0% is approximately20 nm. In the stopband 65B, a central wavelength where transmission isobstructed is 430 nm, and the bandwidth where transmittance is 0% isapproximately 10 nm. In the stopband 65C, a central wavelength wheretransmission is obstructed is 600 nm, and the bandwidth wheretransmittance is 0% is approximately 10 nm. The transmission bands 66A,66B, 66C, and 66D are provided with bandwidths that allow wavelengthsother than these to be transmitted.

According to the absorption filter 13-9 of the present embodiment, it ispossible to form sharp boundaries between the stopbands 65A, 65B, and65C, and the transmission bands 66A, 66B, 66C, and 66D, which are bandsexcluding these stopbands. Therefore, for example, even if thewavelength band of the fluorescent light is close to the wavelength ofthe excitation light, the excitation light and fluorescent light can beexcellently separated. As a result, it is possible to increase theamount of fluorescent light that is transmitted. In addition, it ispossible to more effectively suppress ripples in the transmission bands.Moreover, three stopbands 65A, 65B, and 65C can be set using a singleabsorption filter 13-9.

Next, the tenth embodiment will be described with reference made to FIG.14A. Note that, in the description given below, component elements thatare the same as those in the above described embodiments are given thesame symbols and a description thereof is omitted.

The absorption filter 13-10 of the tenth embodiment is provided with athin film 230. The tenth embodiment differs from the ninth embodiment inthe following points. Namely, in the tenth embodiment, the refractiveindex maximum value in the high refractive index layers of the firstrefractive index profile P1 is different from the refractive indexmaximum value in the high refractive index layers of the secondrefractive index profile P2. In addition, the refractive index minimumvalue in the first refractive index profile P1 is different from therefractive index minimum value in the second refractive index profileP2.

In the present embodiment, as is shown in FIG. 14A, the refractive indexof the substrate 18 is set to 1.9. In addition, in the first refractiveindex profile P1, the refractive indices of the high refractive indexlayers 71A and 72A are changed between 1.9 to 2.1. In contrast, in thefirst refractive index profile P1, the refractive indices of the lowrefractive index layers 71B and 72B are changed between less than 1.9 to1.7. In the second refractive index profile P2, the refractive indicesof the high refractive index layers 73A and 74A are changed between 1.9to 2.2. In contrast, in the second refractive index profile P2, therefractive indices of the low refractive index layers 73B and 74B arechanged between less than 1.9 to 1.6.

Here, the results are shown when a simulation was performed for spectraltransmittance of the absorption filter. In this simulation, there is norefractive index dispersion in the respective layers of the thin film67. The results when transmittance was simulated using these parametersand conditions are shown in FIG. 14B.

This absorption filter 13-10 has the same spectral characteristics asthe absorption filter 13-9 according to the ninth embodiment. Namely,the absorption filter 13-10 is provided with a stopband 76A, a stopband76B, a stopband 76C, and transmission bands 77Aa, 77B, 77C, and 77D. Inthe stopband 76A, the central wavelength is 500 nm. In the stopband 76B,the central wavelength is 430 nm. In the stopband 76C, the centralwavelength is 600 nm. The transmission bands 77A, 77B, 77C, and 77Dallow light of wavelengths other than these to pass through. Here, thebandwidth where the transmittance is 0% in the stopband 76A is 5 nm. Inaddition, the bandwidth of the stopband 76A is narrower than that of thestopband 65A of the absorption filter 13 according to the ninthembodiment.

According to this absorption filter 13-10 of the present embodiment, thesame operation and effects as those from the absorption filter 13-9 ofthe ninth embodiment can be obtained. Moreover, by varying therefractive indices, the width of the stopbands can be optionallychanged.

Note that it is also possible to alter the optical thicknesses in thesecond refractive index profile P2. An example of this is shown in FIG.15A. In FIG. 15A, the optical thicknesses of the low refractive indexlayers 80B and 81B are altered. Specifically, the optical thicknesses ofthe low refractive index layers 80B and 81B are altered from 0.5 timesthe design wavelength to 0.25 times the design wavelength. The spectraltransmittance characteristics of the absorption filter 13-10′ in thiscase are shown in FIG. 15B. In the absorption filter 13-10′ shown inFIG. 15A, in comparison with FIG. 14A, it is possible to move thecentral wavelength of the stopband 83B from 430 nm to 450 nm. Inaddition to this, it is possible to move the central wavelength of thestopband 83C from 600 nm to 570 nm.

FIG. 16A shows another example. In this example, the optical thicknessesof the low refractive index layers 86B and 87B are 0.05 times the designwavelength. The spectral transmittance characteristics of the absorptionfilter 13-10″ in this case are shown in FIG. 16B. In the absorptionfilter 13-10″ shown in FIG. 16A, in comparison with FIG. 14A, it ispossible to move the central wavelength of the stopband 90B from 430 nmto 420 nm. In addition to this, it is possible to move the centralwavelength of the stopband 90C from 570 nm to 550 nm. Moreover, a newstopband 90D can be formed in which the central wavelength is 830 nm,and the bandwidth where the transmittance is 0% is approximately 10 nm.

According to these absorption filters 13-10′ and 13-10″, the sameoperation and effects as in the above described embodiments can beobtained. Moreover, by changing the optical thickness ratio, thepositions of the stopbands can be set to optional positions.

Next, the eleventh embodiment of the present invention will be describedwith reference made to FIG. 17A. Note that, in the description givenbelow, component elements that are the same as those in the abovedescribed embodiments are given the same descriptive symbols and adescription thereof is omitted.

The absorption filter 13-11 of the eleventh embodiment is provided witha thin film 240. The eleventh embodiment differs from the tenthembodiment in the following points. Namely, in the thin film 240 of theabsorption filter 13-11 according to the eleventh embodiment, therefractive indices of the low refractive index layers are substantiallythe same as the refractive index of the substrate.

The thin film 240 is provided with a first refractive index profile P1and a second refractive index profile P2. In addition, a laminatedportion having the first refractive index profile P1 and a laminatedportion having the second refractive index profile P2 are laminatedconsecutively.

The first refractive index profile P1 is represented by a firstlaminated portion 96, a second laminated portion 97, and a thirdlaminated portion 98. The second laminated portion 97 is laminatedbetween the first laminated portion 96 and the third laminated portion98. The second refractive index profile P2 is represented by a firstlaminated portion 103, a second laminated portion 105, and a thirdlaminated portion 106. The second laminated portion 105 is laminatedbetween the first laminated portion 103 and the third laminated portion106.

The thin film 240 is formed by these six laminated portions. From thesubstrate 18 side towards the incident medium 18′, the laminatedportions are formed in order of the third laminated portion 106, thesecond laminated portion 105, the first laminated portion 103, the thirdlaminated portion 98, the second laminated portion 97, and the firstlaminated portion 96. In addition, each laminated portion has lowrefractive index layers and high refractive index layers.

The first refractive index profile P1 will now be described. In thefirst laminated portion 96, the refractive indices of the highrefractive index layers 101 become gradually higher approaching thesubstrate 18 side. In contrast, the refractive indices of the lowrefractive index layers 111 are uniform. In the third laminated portion98, the refractive indices of the high refractive index layers 102become gradually lower approaching the substrate 18 side. In contrast,the refractive indices of the low refractive index layers 111 areuniform. In the second laminated portion 97, the refractive indices ofthe high refractive index layers 100 are uniform. The refractive indicesof the high refractive index layers 100 are substantially the same asthe highest refractive index of the high refractive index layers 101 ofthe first laminated portion 96. Note that the refractive indices of thehigh refractive index layers 100 are substantially the same as thehighest refractive index of the high refractive index layers 102 of thethird laminated portion 98.

The second refractive index profile P2 will now be described. In thefirst laminated portion 103, the refractive indices of the highrefractive index layers 108 become gradually higher approaching thesubstrate 18 side. In contrast, the refractive indices of the lowrefractive index layers 112 are uniform. In the third laminated portion106, the refractive indices of the high refractive index layers 110become gradually lower approaching the substrate 18 side. In contrast,the refractive indices of the low refractive index layers 112 areuniform. In the second laminated portion 105, the refractive indices ofthe high refractive index layers 107 are uniform. The refractive indicesof the high refractive index layers 107 are substantially the same asthe highest refractive index of the high refractive index layers 108 ofthe first laminated portion 103. Note that the refractive indices of thehigh refractive index layers 107 are substantially the same as thehighest refractive index of the high refractive index layers 110 of thethird laminated portion 106.

In addition, the refractive indices of the low refractive index layers111 in the first refractive index profile P1 and the refractive indicesof the low refractive index layers 112 in the second refractive indexprofile P2 are substantially the same as the refractive index of thesubstrate 18.

In the present embodiment, as is shown in FIG. 17A, the refractive indexof the substrate 18 and the refractive indices of the low refractiveindex layers 111 and 112 are set to 1.5. The refractive indices of thehigh refractive index layers 101 and 102 are changed between 1.55 and2.2. In addition, the refractive indices of the high refractive indexlayers 100 are set to 2.2. The refractive indices of the high refractiveindex layers 108 and 110 are changed between 1.55 and 2.4. Therefractive indices of the high refractive index layers 107 are set to2.4.

The total number of the laminated sections having the first refractiveindex profile P1 and the laminated sections having the second refractiveindex profile P2 is 97. Moreover, the design wavelength is 600 nm. Inthe first refractive index profile P1, the optical thicknesses of thehigh refractive index layers 100, 101, and 102 and of the low refractiveindex layers 111 are each 0.25 times the design wavelength. In contrast,in the second refractive index profile P2, the optical thicknesses ofthe high refractive index layers 107, 108, and 110 are each 0.25 timesthe design wavelength, while the optical thicknesses of the lowrefractive index layers 112 are 0.5 times the design wavelength.

Here, the results are shown when a simulation was performed for spectraltransmittance of the absorption filter. In this simulation, there is norefractive index dispersion in the respective layers of the thin film240. The results when transmittance was simulated using these parametersand conditions are shown in FIG. 17B.

This absorption filter 13-11 is provided with a stopband 113A, astopband 113B, and transmission bands 115A, 115B, and 115C. In thestopband 113A, the central wavelength is 600 nm and the bandwidth wheretransmittance is 0% is approximately 120 nm. In the stopband 113B, thecentral wavelength is 450 nm and the bandwidth where transmittance is 0%is approximately 40 nm. The transmission bands 115A, 115B, and 115C areprovided with bandwidths that allow wavelengths other than these to passthrough.

According to this absorption filter 13-11, the same operation andeffects as in the above described embodiments can be obtained. Moreover,by changing the optical thickness ratio, the positions of the stopbandscan be set to optional positions. In addition to this, by changing therefractive indices, the width of the stopbands can be adjusted to anoptional width.

Next, the twelfth embodiment will be described with reference made toFIG. 18A.

The absorption filter 13-12 of the present embodiment is provided with athin film 400. As is shown in FIG. 18A, this thin film 400 is formed bya laminated portion having a first refractive index profile P1 and alaminated portion having a second refractive index profile P2. Eachlaminated portion has low refractive index layers and high refractiveindex layers. These low refractive index layers and high refractiveindex layers are laminated alternatingly.

The first refractive index profile P1 is represented by a firstlaminated portion 300, a second laminated portion 310, and a thirdlaminated portion 320. These three laminated portions are laminated fromthe substrate 18 side in order of the third laminated portion 320, thesecond laminated portion 310, and the first laminated portion 300. Here,the refractive index distributions of each laminated portion are as isdescribed below.

In the first laminated portion 300, the refractive indices of the highrefractive index layers 21 become gradually higher approaching thesubstrate 18. In addition, in the first laminated portion 300, therefractive indices of the low refractive index layers 20 becomegradually lower approaching the substrate 18.

The second laminated portion 310 is adjacent to the first laminatedportion 300. In the second laminated portion 310, the refractive indicesof the high refractive index layers 21 are uniform. The refractiveindices of these high refractive index layers 21 are substantially thesame as the highest refractive index of the high refractive index layers21 that constitute the first laminated portion 300. In addition, in thesecond laminated portion 310, the refractive indices of the lowrefractive index layers 20 are uniform. The refractive indices of thelow refractive index layers 20 are substantially the same as the lowestrefractive index of the first laminated portion 300.

The third laminated portion 320 is adjacent to the second laminatedportion 310. In the third laminated portion 320, the refractive indicesof the high refractive index layers 21 become gradually lower from thesecond laminated portion 310 side. In addition, in the third laminatedportion 320, the refractive indices of the low refractive index layers20 become gradually higher from the second laminated portion 310.

The second refractive index profile P2 has the same profile as that ofthe first refractive index profile P1. The second refractive indexprofile P2 is represented by a fourth laminated portion 330, a fifthlaminated portion 340, and a sixth laminated portion 350. These threelaminated portions are laminated from the substrate 18 side in order ofthe sixth laminated portion 350, the fifth laminated portion 340, andthe fourth laminated portion 330.

The fourth laminated portion 330 has the same refractive indexdistribution as that of the first laminated portion 300. The fifthlaminated portion 340 has the same refractive index distribution as thatof the second laminated portion 310. The sixth laminated portion 350 hasthe same refractive index distribution as that of the third laminatedportion 320.

The low refractive index layers 20 are mainly formed from magnesiumfluoride, while the high refractive index layers 21 are mainly formedfrom niobium oxide.

In the present embodiment, the refractive index of the substrate 18 isset to 1.8. In addition, the refractive indices of the high refractiveindex layers 21 are changed from 1.9 to 2.2. The refractive indices ofthe low refractive index layers 20 are changed from 1.4 to 1.75.

This thin film 400 has refractive index variation layer portions. Forexample, if the vicinity of the boundary between the first laminatedportion 330 and the second laminated portion 310 is observed, it can beseen that a layer 360 is present here that is lower than the highrefractive index layers 21 on both sides thereof. Because this layer 360is on the high refractive index side, here, it is taken as a highrefractive index variation layer portion 360. This high refractive indexvariation layer portion 360 is provided on the first laminated portion330 side.

In the same way, a high refractive index variation layer portion 360 isalso provided in the vicinity of the boundary between the secondlaminated portion 310 and the third laminated portion 320. This highrefractive index variation layer portion 360 is provided on the thirdlaminated portion 320 side. In the present embodiment, the refractiveindices of the high refractive index layers 21 and the second laminatedportion 310 are 2.2. In addition, the refractive index of the highrefractive index variation layer portions 360 are set to 2.1.

In the first and second refractive index profile is P1 and P2, in eachprofile, the central wavelengths of the wavelength bands wheretransmission is blocked are taken as λ1 and λ2. The design wavelength istaken as λ1/n, λ2/m (wherein n and m are integers). At this time, if,for example, n=m=1, the design wavelength become respectively λ1 and λ2.The optical thicknesses of the high refractive index layers 21 and thelow refractive index layers 20 in each refractive index profile are eachset at one quarter of the design wavelength.

In the present embodiment, λ1 is set to 600 nm and λ2 is set to 720 nm.Accordingly, the respective optical thicknesses are 150 nm and 180 nm.

Here, the results are shown when a simulation was performed for spectraltransmittance of the absorption filter. In this simulation, in additionto the above structure, the total number of laminated layers is set at89. In addition, a substrate having a refractive index of 1.8 is alsoprovided on the incident medium 18′ side of the thin film 400. In thissimulation, there is also no refractive index dispersion in each layer.The results of a simulation with these parameters and conditions areshown in FIG. 18B. In FIG. 18B, transmission bands 390 are formed onboth sides of a stopband 380. Here, the stopband 380 is the waveband towhich, for example, excitation light and the like belongs. Accordingly,in this waveband, excitation light is prevented from being irradiatedonto the observation side (i.e., onto the ocular lens 14 shown in FIG.1). In contrast, the transmission bands 390 are wavebands to whichfluorescent light belongs. Accordingly, in these wavebands, fluorescentlight is allowed to pass through to the observation side.

According to this absorption filter 13-12, for example, as is shown inFIG. 18B, it is possible to widen the stopband 380. This is because therefractive index distribution is the same and also because a pluralityof lamination patterns of different optical thicknesses are employed.Moreover, the first refractive index profile P1 and the secondrefractive index profile P2 only have different optical thicknesses fromeach other, and the refractive index distributions of the respectiverefractive index profiles are the same. Consequently, the film structureallows control to be simplified during film formation. As a result, itis possible to improve the consistency of the optical characteristics.

Next, the thirteenth embodiment will be described with reference made toFIG. 19A. Note that, in the description given below, component elementsthat are the same as those in the above described embodiments are giventhe same symbols and a description thereof is omitted.

The absorption filter 13-13 of the thirteenth embodiment differs fromthat of the twelfth embodiment in the following points. Namely, in thatthe wavelength λ1 in the first refractive index profile P1 is 480 nm,and in that the optical thickness thereof is 120 nm.

In the present embodiment as well, the total number of laminated layersof the thin film 410 is 89. In addition, a substrate having a refractiveindex of 1.8 is also provided on the incident medium 18′ side of thethin film 410. In this simulation, there is also no refractive indexdispersion in each layer. The results of a simulation with theseparameters and conditions are shown in FIG. 19B.

According to the absorption filter of the present embodiment, as isshown in FIG. 19B, a stopband 380A is formed by the first refractiveindex profile P1 and a stopband 380B is formed by the second refractiveindex profile P2. At this time, the stopband 380A and the stopband 380Bare formed in separated positions. Accordingly, it is possible to obtainfilter characteristics having a transmission region 390 between the two.

Next, the fourteenth embodiment will be described with reference made toFIG. 20A. Note that, in the description given below, component elementsthat are the same as those in the above described embodiments are giventhe same symbols and a description thereof is omitted.

The absorption filter 13-14 of the fourteenth embodiment differs fromthat of the twelfth embodiment in the following points. Namely, in thethin film 420 of the fourteenth embodiment, the refractive indices ofthe low refractive index layers 20 are uniformly set to the same valueas the refractive index of the substrate 18. This is the same in boththe first refractive index profile P1 and the second refractive indexprofile P2. The absorption filter 13-14 also differs in that there areno refractive index variation layer portions 360.

Namely, the thin film 420 is formed by first through sixth laminatedportions 300 to 350, however, the refractive indices of the lowrefractive index layers 20 that constitute each laminated portion areset to the same value as the refractive index of the substrate 18.

In the present embodiment, the refractive index of the substrate 18 isset to 1.5. The refractive indices of the high refractive index layers21 are changed from 1.55 to 2.4. In addition, the refractive indices ofthe low refractive index layers 20 are set to a uniform value of 1.5.

Here, the results are shown when a simulation was performed for spectraltransmittance of the absorption filter. In this simulation, in additionto the above structure, the total number of laminated layers is set at93. In addition, a substrate having a refractive index of 1.5 is alsoprovided on the incident medium 18′ side of the thin film 420. In thissimulation, there is also no refractive index dispersion in each layer.The results of a simulation with these parameters and conditions areshown in FIG. 20B.

According to the absorption filter of the present embodiment as well,for example as is shown in FIG. 20B, it is possible to obtain a widestopband 380 in the same way as in the above described first embodiment.Namely, it is possible to obtain filter characteristics in which thestopband of the first refractive index profile P1 and the stopband ofthe second refractive index profile P2 are combined.

Note that, in the above described embodiment, n=m=1. In addition, thedesign wavelength is the same as the central wavelength in eachlamination pattern. Moreover, the optical thicknesses of the highrefractive index layers 21 and the low refractive index layers 20 areset to one quarter of the design wavelength. However, it is alsopossible for n=m=2. In this case, the optical thicknesses of the highrefractive index layers 21 and the low refractive index layers 20 areset to one half of the design wavelength. It is possible to obtain aspectral filter having exactly the same spectral characteristics even ifa thin film is formed under these conditions.

Furthermore, it is also possible to use a design wavelength of 600/n(wherein n is an integer) nm for a central wavelength of 600 nm. Inaddition, the optical thicknesses of the high refractive index layers 21and the low refractive index layers 20 are set at n/4 of the designwavelength. Even when a thin film is formed under these conditions, itis still possible to obtain an absorption filter having exactly the samespectral characteristics.

Next, the fifteenth embodiment of the present invention will bedescribed with reference made to FIG. 21A.

The absorption filter 13-15 is formed by a glass substrate 18, and by athin film 500 that is formed on top of the substrate 18. This absorptionfilter 13-15 differs from the absorption filters previously described inthat there is no incident side medium 18′. Due to this structure, theabsorption filter 13-15 of the present embodiment selectively allowsonly the above described fluorescent light to pass through.

As is shown in FIG. 21A, the thin film 500 is formed by an outermostlayer portion 600 and a portion having a refractive index profile 610.The portion having the refractive index profile 610 has low refractiveindex layers and high refractive index layers. In the portion having therefractive index profile 610, these low refractive index layers and highrefractive index layers are laminated alternatingly.

The outermost layer portion 600 is in contact with air (i.e., an opticalmedium) 18″ whose refractive index is lower than that of the substrate18. The refractive indices of the low refractive index layers 20 of theportion having the refractive index profile 610 are higher than therefractive index of the air 18″ and equal to or lower than therefractive index of the substrate 18. The refractive indices of the highrefractive index layers 21 of the portion having the refractive indexprofile 610 are relatively higher than the refractive indices of the lowrefractive index layers 20.

The outermost layer portion 600 will now be described. As is describedabove, the outermost layer portion 600 is in contact with the air 18″.This outermost layer 600 has an outermost low refractive index layer600A, a first outermost high refractive index layer 600B, and a secondoutermost high refractive index layer 600C.

In the outermost low refractive index layer 600A, the refractive indexis set to be higher than the refractive index of air and equal to orlower than the refractive index of the substrate. The first outermosthigh refractive index layer 600B is laminated onto the outermost lowrefractive index layer 600A. In the first outermost high refractiveindex layer 600B, the refractive index is set to be higher than therefractive index of the outermost low refractive index layer 600A. Thesecond outermost high refractive index layer 600C is laminated onto thefirst high refractive index layer 600B. In the second outermost highrefractive index layer 600C, the refractive index is set to be higherthan the refractive index of the first outermost high refractive indexlayer 600B.

The refractive index profile P will now be described. The refractiveindex profile P is represented by a first laminated portion 610A, asecond laminated portion 610B, and a third laminated portion 610C.

The first laminated portion 610A is laminated on the substrate 18 sideof the outermost layer portion 600. In the first laminated portion 610A,low refractive index layers 20 are adjacent to the outermost layerportion 600. In addition, high refractive index layers 21 are adjacentto the outermost layer portion 600. In the first laminated portion 610A,the refractive indices of the high refractive index layers 21 are higherthan the refractive index of the second outermost high refractive indexlayer 600C. Moreover, in the first laminated portion 610A, therefractive indices of the high refractive index layers 21 becomegradually higher approaching the substrate 18 side. In addition, in thefirst laminated portion 610A, the refractive indices of the lowrefractive index layers 20 are uniform. The refractive indices of thelow refractive index layers 20 are substantially the same as therefractive index of the outermost low refractive index layer 600A.

The third laminated portion 610C is laminated on the substrate 18 sideof the first laminated portion 610A. In the third laminated portion610C, the refractive indices of the high refractive index layers 21become gradually lower approaching the substrate 18 side. In addition,in the third laminated portion 610C, the refractive indices of the lowrefractive index layers 20 become gradually higher approaching thesubstrate 18 side. Note that the refractive indices of the lowrefractive index layers 20 ultimately become substantially the same asthe refractive index of the substrate 18.

The second laminated portion 610B is laminated between the firstlaminated portion 610A and the third laminated portion 610C. In thesecond laminated portion 610B, the refractive indices of the highrefractive index layers 21 are uniform. The refractive indices of thehigh refractive index layers 21 are substantially the same as thehighest refractive index of the high refractive index layers 21 in thefirst laminated portion 610A. In addition, in the second laminatedportion 610B, the refractive indices of the low refractive index layers20 are uniform. The refractive indices of the low refractive indexlayers 20 are substantially the same as the refractive index of theoutermost low refractive index layer 600A.

Here, the low refractive index layers 20 are mainly formed frommagnesium fluoride, while the high refractive index layers 21 are mainlyformed from tantalum oxide.

In the present embodiment, the refractive index of the substrate 18 isset to 1.52. In addition, the refractive index of the outermost lowrefractive index layer 600A in the outermost layer portion 600 is set to1.4, while the refractive indices of the first outermost high refractiveindex layer 600B and the second outermost high refractive index layer600C of the outermost layer portion 600 are set respectively to 1.5 and1.6.

Moreover, in the first laminated portion 610A, the refractive indices ofthe high refractive index layers 21 are changed from 1.7 to 2.2. Inaddition, in the third laminated portion 610C, the refractive indices ofthe high refractive index layers 21 are changed from 1.52 to 2.2. In thefirst laminated portion 610A and the second laminated portion 610B, therefractive indices of the low refractive index layers 20 are set to 1.4.In addition, in the third laminated portion 610C the refractive indicesof the low refractive index layers 20 are changed from 1.4 to less than1.52.

In the thin film 500 of the present embodiment, the total number oflaminated layers is 45. This number is the total number from thesubstrate 18 side to the outermost low refractive index layer 600A ofthe outermost layer portion 600. The design wavelength is 600 nm. Theoptical thickness of each layer is 0.25 times the design wavelength.

In the simulation, there was no refractive index dispersion in therespective layers. The results of a simulation with these parameters andconditions are shown in FIG. 21B.

This absorption filter 13-15 is provided with a stopband 700 andtransmission bands 710A and 710B. In the stopband 700, the centralwavelength where transmission is obstructed is approximately 610 nm. Inaddition, in the stopband 700, the bandwidth where transmittance isapproximately 0% is approximately 140 nm. The transmission bands 710Aand 710B allow wavelengths other than these to be transmitted.

According to this absorption filter 13-15, for example, as is shown inFIG. 21B, it is possible to form sharp boundaries between the stopband700 and transmission bands 710A and 710B. As a result, the amount oflight transmitted in the transmission bands 710A and 710B can beincreased. Furthermore, it is possible to suppress the generation ofripples in the vicinity of the boundaries. Moreover, because the filmstructure simplifies control of the optical thickness during filmformation, it is possible to improve the consistency of the opticalcharacteristics.

Furthermore, as is shown in FIG. 22, the absorption filter 13-15 hasoptical characteristics that are close to those of an ideal filter.Therefore, if this absorption filter 13-15 is used in the fluorescencemicroscope 10, the detection sensitivity when measuring fluorescentlight can be improved considerably. Namely, in a conventional filter,the transmittance is reduced in wavelength regions that are close toexcitation light from among the wavelength regions of fluorescent light.As a result, in the conventional filter, the amount of transmittedfluorescent light is reduced. However, in the filter of the presentembodiment, it is possible to transmit the amount of light of thesewavelength regions (i.e., the increased portion of the amount of light)at a high transmittance.

As a result, for example, in genome analysis and the like, it ispossible to improve analysis accuracy and detection accuracy, and toshorten the observation time.

Moreover, as described above, in the filter of the present embodiment,the refractive indices of the low refractive index layers 20 arelaminated as they change. Accordingly, it is possible to suppress lossat boundaries between the substrate 18 and the thin film 500. As aresult, it is possible to further improve the amount of lighttransmitted in the transmission bands 710A and 710B.

Note that the absorption filter may also be formed like the absorptionfilter 13-15′ shown in FIG. 23A. Here, the refractive index profile ofthe thin film 511 is changed. Specifically, in the first laminatedportion 611A, the total number of laminated layers is increased from 12to 28. As a result, the rate of change of the high refractive indexlayers 21 is a more gently sloping rate of change than that of the firstlaminated portion 610A shown in FIG. 21A. As is shown in FIG. 23B, it ispossible to obtain substantially the same spectral transmittancecharacteristics as those shown in FIG. 21B even when this type ofstructure is employed. Namely, it is possible to form a stopband 20 thatis provided with the same central wavelength and bandwidth as that shownin FIG. 21B. FIG. 24A and FIG. 24B are comparative examples. In FIG.24A, the outermost layer portion 601 is not provided. In thiscomparative example, ripples are generated in the vicinity of theboundaries. Compared with the case shown in FIG. 24A, in the absorptionfilter shown in FIG. 23A, the same operation and effects as those shownin FIG. 21A can be obtained.

Note that the absorption filter may also be formed like the absorptionfilter 13-15″ shown in FIG. 25A. Here, a different refractive indexprofile P is formed. Specifically, the total number of laminated layersof the first laminated portion 612A is decreased from 12 to 4. As aresult, the rate of change of the high refractive index layers 21 is amore abruptly sloping rate of change than that of the first laminatedportion 18A shown in FIG. 21A. As is shown in FIG. 25B, it is possibleto obtain substantially the same spectral transmittance characteristicsas those shown in FIG. 21B even when this type of structure is employed.FIG. 26A and FIG. 26B are comparative examples in which the outermostlayer portion 15 is not provided. In this comparative example as well,ripples are generated in the vicinity of the boundaries. Compared withthe case shown in FIG. 26A, in the absorption filter shown in FIG. 25A,the same operation and effects as those shown in FIG. 21A can beobtained.

Next, the sixteenth embodiment of the present invention will bedescribed with reference made to FIG. 27A. Note that, in the descriptiongiven below, component elements that are the same as those in the abovedescribed embodiments are given the same symbols and a descriptionthereof is omitted.

The sixteenth embodiment differs from the fifteenth embodiment in thefollowing points. Namely, in the fifteenth embodiment, there is only onerefractive index profile in the substrate 18, however, in the sixteenthembodiment there are two refractive index profiles in the substrate 18.Specifically, as a refractive index profile P, the thin film 510 of theabsorption filter 13-16 is provided with a first refractive indexprofile P1 and a second refractive index profile P2. In addition,laminated portions having these profiles are laminated adjacent to eachother in sequence approaching the substrate 18 side.

Namely, in the thin film 510, as is shown in FIG. 27A, a laminatedportion having the second refractive index profile P2 is laminated onthe substrate 18. A laminated portion having the first refractive indexprofile P1 is then laminated on top of that, and an outermost layerportion 600 is then further laminated on top of that.

Moreover, the optical thicknesses of each layer in the outermost layerportion 600 and the optical thicknesses of each layer in the firstrefractive index profile P1 are different from the optical thicknessesof each layer in the second refractive index profile P2. Note that theterm “each layer in the first refractive index profile P1” refers to thehigh refractive index layers 21 and the low refractive index layers 20.In addition, the term “each layer in the second refractive index profileP2” refers to the high refractive index layers 21′ and the lowrefractive index layers 20′.

In the present embodiment, the refractive index of the substrate 18 is1.52, which is the same as in the fifteenth embodiment. In addition, thefirst refractive index profile P1 is represented by a first laminatedportion 620A, a second laminated portion 620B, and a third laminatedportion 620C. Note that the first refractive index profile P1 is thesame as the refractive index profile P in the fifteenth embodiment.Namely, the refractive indices of the first laminated portion 620A, thesecond laminated portion 620B, and the third laminated portion 620C arethe same as those of the first laminated portion 610A, the secondlaminated portion 610B, and the third laminated portion 610C.

The second refractive index profile P2 is represented by a firstlaminated portion 630A, a second laminated portion 630B, and a thirdlaminated portion 630C. Here, in the first laminated portion 630A andthe third laminated portion 630C, the refractive indices of the lowrefractive index layers 20′ change between less than 1.52 and 1.4. Inaddition, in the second laminated portion 630B, the refractive indicesof the low refractive index layers 20′ are set to 1.4. Moreover, in thesecond laminated portion 630B, the refractive indices of the highrefractive index layers 21′ are changed between 1.52 and 2.2.

In the thin film 510 of the present embodiment, the total number oflaminated layers is 89. This number is the total number from thesubstrate 18 to the outermost low refractive index layer 600A of theoutermost layer portion 600. The design wavelength is 600 nm. Theoptical thicknesses of each layer in the first refractive index profileP1 are 0.25 times the design wavelength. The optical thicknesses of eachlayer in the second refractive index profile P2 are 0.3 times the designwavelength.

In the simulation, there is no refractive index dispersion in therespective layers of the thin film 510. The results of the simulationusing these parameters and conditions are shown in FIG. 27B.

This absorption filter 13-16 is provided with a stopband 720 andtransmission bands 730A and 730B. In the stopband 720, the centralwavelength where transmission is obstructed is substantially 680 nm. Inaddition, in the stopband 720, the bandwidth where transmittance issubstantially 0% is approximately 280 nm. The transmission bands 730Aand 730B allow light of wavelengths other than these to pass through.

FIG. 28A and FIG. 28B show a case in which the outermost layer portion600 is not provided. As can be seen in a comparison with FIG. 28A,according to the absorption filter 13-16 of the present embodiment, inthe same way as in the first embodiment, it is possible to reduceripples in the fluorescent light transmission bands 730A and 730B. Inaddition, according to this absorption filter 13-16, it is possible toconsistently obtain a sufficient amount of light. Moreover, it is alsopossible to change the optical thicknesses of each layer between theoutermost layer portion 600 and first refractive index profile P1 andthe second refractive index profile P2. By employing this type ofstructure, the position of the central wavelength of the stopband 720can be moved to an optional position. In addition to this, the width ofthe stopband 720 can be set to an optional size.

Next, the 17th embodiment of the present invention will be describedwith reference made to FIG. 29A. Note that, in the description givenbelow, component elements that are the same as those in the fifteenthembodiment are given the same symbols and a description thereof isomitted.

The seventeenth embodiment differs from the fifteenth embodiment in thefollowing point. Namely, in that, in the absorption filter 13-17 of theseventeenth embodiment, the refractive indices of each layer of anoutermost layer portion 640 of a thin film 520 and of each layer in arefractive index profile P are different from those in the fifteenthembodiment.

Namely, in the present embodiment, as is shown in FIG. 29A, in theoutermost layer portion 640, the refractive index of the outermost lowrefractive index layer 640A is set to 1.46, the refractive index of thefirst outermost high refractive index layer 640B is set to 1.56, and therefractive index of the second outermost high refractive index layer640C is set to 1.66.

Moreover, in a first laminated portion 650A, the refractive indices ofthe high refractive index layers 21 are changed from 1.76 to 2.29. Inaddition, in a third laminated portion 650C, the refractive indices ofthe high refractive index layers 21 are changed from 1.52 to 2.29. Inthe first laminated portion 650A and the second laminated portion 650B,the refractive indices of the low refractive index layers 20 are set to1.46. In the third laminated portion 650C, the refractive indices of thelow refractive index layers 20 are changed between 1.46 and less than0.1.52.

In this thin film 520, the total number of laminated layers is 44. Thisnumber is the total number of layers from the substrate 18 side to theoutermost low refractive index layer 640A of the outermost layer portion640. The design wavelength is 600 nm. The optical thicknesses of eachlayer are 0.25 times the design wavelength.

Here, the low refractive index layers 20 are mainly formed from siliconoxide, while the high refractive index layers 21 are mainly formed fromtitanium oxide.

In the simulation, there is no refractive index dispersion in therespective layers of the thin film 520. The results of the simulationusing these parameters and conditions are shown in FIG. 29B.

This absorption filter 13-17 is provided with a stopband 750 andtransmission bands 750A and 750B. In the stopband 750, the centralwavelength where transmission is obstructed is substantially 610 nm. Inaddition, in the stopband 750, the bandwidth where transmittance issubstantially 0% is approximately 140 nm. The transmission bands 750Aand 750B allow light of wavelengths other than these to pass through.

FIG. 30A and FIG. 30B show a case in which the outermost layer portion640 is not provided. As can be seen in a comparison with FIG. 30A,according to this absorption filter 13-17, the same operation andeffects as those obtained in the first embodiment can be obtained whenthe refractive indices of each layer are different from the refractiveindices in the fifteenth embodiment. Namely, according to thisabsorption filter 13-17 as well, it is possible to reduce ripples in thetransmission bands 750A and 750B.

Moreover, compared with the constituent materials of each layer in thefifteenth embodiment, because the constituent materials of the presentembodiment are easily applied to spattering, the degree of freedom inthe film formation process can be increased.

Next, the eighteenth embodiment of the present invention will bedescribed with reference made to FIG. 31A. Note that, in the descriptiongiven below, component elements that are the same as those in the abovedescribed embodiments are given the same symbols and a descriptionthereof is omitted.

The eighteenth embodiment differs from the seventeenth embodiment in thefollowing point. Namely, as is shown in FIG. 31A, in the thin film 530of the absorption filter 13-18 of the eighteenth embodiment, theoutermost layer portion 660 is not in contact with air 13, but is incontact with glass (i.e., an optical medium) 18′″ having a refractiveindex of 1.46.

In addition, in the present embodiment, the difference with therefractive index of the glass 18′″ is clearly evident. Therefore, in anoutermost low refractive index layer 660A and in a first laminatedportion 670A and second laminated portion 670B, the refractive indicesof the low refractive index layers 20 are set to 1.67. In addition, in athird laminated portion 670C, the refractive indices of the lowrefractive index layers 20 are changed between 1.67 and 2.29.

Furthermore, the refractive index of a first outermost high refractiveindex layer 660B is set to 1.72, and the refractive index of a secondoutermost high refractive index layer 660C is set to 1.8. In addition,in the first laminated portion 670A, the refractive indices of the highrefractive index layers 21 are changed between 1.8 and 2.29.

In the thin film 530, the total number of laminated layers is 44. Thisnumber is the total number of layers from the substrate 18 side to theoutermost low refractive index layer 660A of the outermost layer portion660. The design wavelength is 600 nm. The optical thicknesses of eachlayer are 0.25 times the design wavelength.

Here, the outermost low refractive index layer 660A and the lowrefractive index layers 20 are mainly formed from alumina.

In the simulation, there is no refractive index dispersion in therespective layers of the thin film 530. The results of the simulationusing these parameters and conditions are shown in FIG. 31B.

This absorption filter 13-18 is provided with a stopband 760 andtransmission bands 770A and 770B. In the stopband 760, the centralwavelength is substantially 610 nm. In addition, in the stopband 760,the bandwidth where transmittance is substantially 0% is approximately90 nm. The transmission bands 770A and 770B allow light of wavelengthsother than these to pass through.

FIG. 32A and FIG. 32B show a case in which the outermost layer portion660 is not provided. As can be seen in a comparison with FIG. 32A,according to this absorption filter 13-18, the same operation andeffects as those obtained in the seventeenth embodiment can be obtainedeven if the outermost layer portion 660 is in contact with an opticalmedium other than air such as the glass 18′″.

Note that the technological range of the present invention is notlimited by the above described embodiments, and various modificationscan be made without departing from the spirit or scope of the presentinvention.

For example, the central wavelength (λ) is not limited to 600 nm, and byappropriately changing the value of λ in accordance with the wavelengthof the excitation light and the wavelength of the fluorescent light tobe detected, it is possible to obtain the desired spectralcharacteristics.

In addition, the material of the substrate is not limited to glass, andplastic may also be used.

1. An optical filter formed by a substrate and a thin film that isformed on the substrate, wherein the thin film is formed by laminatinglow refractive index layers whose refractive index is relatively lowalternatingly from the substrate side with high refractive index layerswhose refractive index is relatively high, and wherein the thin film isprovided with a first laminated portion, a second laminated portion thatis adjacent to the first laminated portion, and a third laminatedportion that is adjacent to the second laminated portion, and wherein inthe first laminated portion, the refractive indices of the highrefractive index layers become gradually higher approaching thesubstrate, in the second laminated portion, the refractive indices ofthe high refractive index layers are substantially equal to the highestrefractive index of the high refractive index layers constituting thefirst laminated portion, and in the third laminated portion, therefractive indices of the high refractive index layers become graduallylower from the second laminated portion side, and wherein the refractiveindices of the low refractive index layers constituting the firstthrough third laminated portions are substantially equal to therefractive index of the substrate, and an absolute value of a refractiveindex gradient of the high refractive index layers constituting thefirst laminated portion is different from an absolute value of arefractive index gradient of the high refractive index layersconstituting the third laminated portion.
 2. The optical filteraccording to claim 1, wherein when a design wavelength is set to λ/n(wherein n is an integer) for a central wavelength (λ) of a wavelengthband where transmission is obstructed, optical thicknesses of the highrefractive index layers and the low refractive index layers are set tosubstantially n/4 of the design wavelength.